1. Field of the Invention
The present invention is directed to a radio-frequency reception arrangement for a magnetic resonance apparatus of the type having a number of independent antennas and pre-amplifiers.
2. Description of the Prior Art
For producing an image by magnetic resonance, it is often advantageous to simultaneously detect the radio-frequency magnetic resonance signal with a number of reception antennas. The image quality that can be obtained thereby generally increases with the number of independent reception antennas that are available.
For more recent applications such as, for example, peripheral angiography, there are multi-element antenna arrangements wherein the number of individual antennas that are applied to the patient is higher than the number of independent radio-frequency receiver channels. Given such multi-element antenna arrangements, the possible imaging area (field of view) of all antennas applied to the patient exceeds the maximum imaging area that is predetermined by the homogeneity of the main magnet. The scans are made in chronological succession along a number of regions, which may partially overlap. Dependent on the application, the patient table also can be displaced. The advantage of such a multi-element antenna arrangement is that the exposures (scans) can ensue in rapid succession without having to re-position the antennas at the patient or the patient on the table.
U.S. Pat. No. 4,825,162 discloses a radio-frequency arrangement of the type initially described. This includes an array of a number of independent local antennas that are respectively connected to a low-noise pre-amplifier. For decoupling the local antennas from one another, local antennas arranged neighboring one another partially overlap and the input impedance of the pre-amplifiers and matching elements required for line adaptation are dimensioned such that the input impedance of the pre-amplifier transformed at the antenna terminal is seen as a high-impedance. This electronic decoupling prevents a current from flowing in the antenna as a consequence of an induced voltage. The magnetic field associated with such a current would otherwise induce signals in a neighboring antenna. Even though there are still inductive couplings due to the geometrical arrangement, antennas that are not used in the imaging cannot feed signals nor additional patient noise from neighboring antennas into active antennas.
Due to the aforementioned electronic decoupling, the individual antennas each are permanently connected to an allocated, low-noise pre-amplifier.
German PS 197 09 244 discloses a switching matrix that can merge high-frequency signals such as, for example, MR signals in-phase. It is thus possible to sum the reception signals of a number of antennas and then supply the sum to a single radio-frequency reception channel.
An object of the present invention is to provide a radio-frequency antenna arrangement for a magnetic resonance apparatus wherein the number of independent antennas is greater than the number of signal processing channels, with less outlay.
This object is achieved in accordance with the invention in a radio-frequency antenna arrangement for a magnetic resonance apparatus wherein the number of pre-amplifiers is less than the number of independent antennas, and wherein a switching matrix is arranged between the pre-amplifiers and the antennas for selective, signal-dependent connection of the pre-amplifiers to the antennas. The number of pre-amplifiers thus can be matched to the number of existing signal processing channels. The not inconsiderable outlay for the pre-amplifiers can be substantially reduced given multi-element antenna arrangements with a significantly higher number of individual antennas than existing signal processing channels.
The antennas that are not connected to the pre-amplifiers can, for example, be actively detuned for decoupling from the active antennas. In an embodiment, however, simulations of an input impedance of the pre-amplifiers are connected to the antennas, these simulations being connectable in signal-dependent terms to the antennas with the switching matrix. The advantages of the pre-amplifier decoupling thus can be utilized; an active detuning of the coils that are not used for readout at the moment, and the control outlay corresponding thereto, thus can be eliminated.
In a further embodiment the switching matrix is fashioned for connecting each antenna to each pre-amplifier in signal-dependent terms. The wide-ranging freedom in the connection of the antennas to the pre-amplifiers that is thus established can be used for an even greater reduction in the number of pre-amplifiers.
High-frequency analog switches in, for example, GaAs field effect transistor technology can be utilized as high-frequency switches in the switching matrix. Care should thereby be exercised in the design to ensure that no undesired parasite tendency arises. A less critical design is possible to another embodiment wherein the switching matrix employs high-frequency switching diodes, for example PIN diodes, as switching elements.